The invention relates to a predistortion linearizer circuit and a method on which it is based. The predistortion linearizer circuit of the invention is primarily intended to be used to predistort the amplitude and phase of the input signal of a travelling wave tube amplifier in order to cancel the amplitude and phase distortion caused by the amplifier. To be more precise, the purpose of the predistortion linearizer circuit of the invention is that the third and fifth order intermodulation distortion, AM/AM and AM/PM response, AM/PM transition and noise/power ratio of the linearized travelling wave tube amplifier can be substantially minimized.
Prior art predistortion linearizer circuits have been implemented by means of one or two non-linear components (a diode or a transistor, for instance) and by dividing a signal into two branches: linear and non-linear. The former solution utilizes the increase of transmission loss or reflection of the non-linear component as a function of the input power. In the latter solution, the non-linear branch being compressed is added to the linear branch by an appropriate phase shift and amplitude. A predistortion linearizer circuit operating at a certain frequency can be formed in both ways. A predistortion linearizer circuit of this kind is disclosed in U.S. Pat. No. 4,752,743, for instance. A predistortion linearizer circuit PDL can be used in accordance with FIG. 1, for example, to predistort the input signal of a travelling wave tube amplifier TWTA. The predistortion linearizer circuit consists of two main parts: an amplitude distorting part AM/AM and a phase distorting part AM/PM. (The two-part term AM/PM is meant to indicate that amplitude modulation of the input signal causes phase modulation (unit xc2x0/dB) of the output signal. In addition, the predistortion linearizer circuit PDL comprises a mechanism for inhibiting direct current (and a modulation frequency signal). In FIG. 1, this kind of mechanism is a capacitor C11 coupled in series with the signal, but it can also be a transformer (not shown), etc.
The problem in the prior art predistortion linearizer circuits is that they operate only in a narrow frequency band. If the operating frequency is changed, the prior art predistortion linearizer circuits must be tuned to the new operating frequency. In other words, they are not sufficiently broadband.
An object of the invention is to provide a predistortion linearizer circuit that operates in a broader frequency band than the prior art predistortion linearizer circuits. The objects of the invention can be achieved with a method and a predistortion linearizer circuit which are characterized by what is stated in the characterizing parts of the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the observation that the phase-frequency distortions of a non-linear transmission line with a certain transit time and biased by appropriate biasing means and of a travelling wave tube amplifier as a function of the input power are inversely related over a broad frequency band. The basis of the implementation of the invention is that at least the phase distorting part of the predistortion linearizer circuit comprises a non-linear transmission line and broadband biasing means for appropriately biasing the non-linear transmission line to an operating point changing with incoming amplitude modulation. In this connection, the broadband biasing means imply that at a modulation bandwidth their input impedance is designed to be suitable for detecting the modulation, but at a carrier frequency their input impedance is high. The structure is thus of the type of a low-pass filter. The transmission line comprises several stages, each comprising an inductance coupled in series with the signal and a capacitance coupled in parallel with the signal. In practice, the transmission line is formed by a component arrangement satisfying the characteristic transmission line equations known to those skilled in the art, as will be described below. The minimum number of the stages is two; in practice about 20 stages are necessary. A non-linear transmission line means that an inductance and/or a capacitance (in practice capacitance) changes as a function of the instantaneous voltage or currency of an input signal. The carrier wave power must thus be detected and the transmission line voltage in the modulation band must be controlled by the detected signal. The practical implementation of the non-linear transmission line is usually an inductance in series with the signal and an appropriately biased semiconductor junction, such as a diode, in parallel with the signal.
The predistortion linearizer circuit of the invention is a broadband one. Its phase-frequency distortion in a broad frequency band is inverse to the phase-frequency distortion caused by a travelling wave tube. The propagation delay of the travelling wave tube becomes longer with the increase of the input power, while the propagation delay of the predistortion linearizer circuit of the invention correspondingly becomes shorter, so the phase-frequency distortions of the predistortion linearizer circuit and travelling wave tube cancel each other out over the broad frequency band.
The active part of the predistortion linearizer circuit of the invention is an extremely simple non-linear transmission line. In practice the transmission line is implemented by two components, one of which can be merely a short bit of wire. This makes diverse modifications possible without making the whole circuit considerably more complex. The non-linear transmission line is known from other contexts, so those skilled in the art can easily manage its implementation, design and behaviour.